Directional microphones are an effective way to facilitate the comprehension of voice in an environment full of interfering sound since they have a sensitivity depending on the direction of the incidence of sound (directional pattern) and, thus, produce a spatial suppression of interfering sounds.
Directional pattern or directional effect describes the ratio of the sensitivities of a microphone to sound sources impinging on the microphone from all directions of one plane and essentially depends on the construction of the microphone. Known directional patterns are spherical or omnidirectional, figure-of-eight or bidirectional, cardioid, supercardioid, hypercardioid and lobe pattern.
The spherical pattern is distinguished by the fact that the sound is picked up with the same strength from all directions. A microphone having a spherical pattern is, for example, the “pressure transducer”, the diaphragm of which, only the front of which is exposed to the sound field, picks up all pressure fluctuations located in the sound field regardless of the direction from which they come. Since this microphone does not have a preferred directional effect, it has a spherical pattern and is frequently called a “spherical microphone”.
A figure-of-eight pattern is distinguished by the fact that the sound is picked up with particular intensity from two selected directions which are opposite to one another. Microphones having a figure-of-eight pattern, also called “figure-of-eight microphones”, have been developed for, among other things, the M/S stereo method and enable the stereo base to be subsequently influenced right up to mono.
A microphone having a figure-of-eight pattern is, e.g., the “pressure-gradient transducer” or “pressure-difference transducer” which is designed in such a way that the sound reaches the diaphragm both from the front and from the back, which requires two sound entry openings so that the diaphragm is not deflected when sound arrives from the side and a “figure-of-eight” directional pattern is guaranteed.
A further possibility of achieving a figure-of-eight pattern which, moreover, is more flexible than the purely mechanical arrangement of the pressure-gradient transducer, is an arrangement of two simple spherical microphones which are slightly offset in space (array). The directional effect is obtained by electronically subtracting the spherical-microphone signal at the front (from the point of view of the incident sound) from the delayed signal of the spherical microphone at the rear. The precise shape of the directional pattern is defined by the microphone spacing and the internal electrical delay.
The pressure-difference or pressure-gradient transducer supplies a signal proportional to cos(α) with a sound incident at an angle α and is, therefore, a directional microphone having a first-order directional pattern.
Dispensing with close-talking microphones in telephones, in video conferences or in automatic voice recognition leads to reverberation and background noises becoming superimposed on voice. These unwanted signal components are compensated for by using a directional microphone having one of the patterns mentioned above, particularly via a controllable (directional-) microphone array, the main lobe of which is focused on the speaker, typically automatically.
In this context, “controllable” refers to the direction (orientation) of the main lobe, which is determined by an angle (φ) which is preset or automatically orientated toward a speaker by methods of localization and voice detection (i.e., is variable), being adjustable by, in particular digital, signal postprocessing of the received signals generated by the directional microphones from an incident sound.
Therefore, a controllable first-order directional microphone is obtained in a familiar manner when a signal generated by a first-order directional microphone (e.g., pressure-difference transducer) is postprocessed via signal processing so that a desired direction (φ) of the main lobe is imparted to the signal and, finally, a signal results which is proportional to cos(φ+α).
However, directional-microphone arrangements with a second-order directional pattern, particularly controllable directional-microphone arrangements, are not known.
An object of the present invention is, therefore, to specify a system and a method which ensure a, particularly controllable, second-order directional-microphone pattern.